The ability of face masks to reduce transmission of microbes

Optometrists have been advised to wear face masks during the COVID-19 pandemic. This study examined whether face masks were equally protective against transmission of microbes.

The aim of the current study was to examine the ability of face masks to reduce transmission of microbes in aerosols and during speech.

Different face masks, surgical, medical 3-ply and cloth masks with different layers were used. The masks were tested under the ASTM standard F2101-1 to measure their ability to reduce the transmission of aerosolised Staphylococcus aureus. Bacterial cells in different sized aerosols were captured on agar plates. The ability of masks to reduce the transmission of bacteria during speech over 30 cm was measured. Bacteria were captured in masks or on agar plates at a distance of 30 cm during the speech.

All masks reduced the transmission of aerosolised S. aureus (p ≤ 0.007). The medical 3-ply and cloth masks with three layers reduced the transmission of S. aureus aerosols (3.3 μm) by 98% and surgical and seven-layer masks reduced this by 100%. An antibacterial silver mask showed significantly greater transmission of S. aureus in aerosols of 4.7 μm (16 ± 6 cells) and 3.3 μm (122 ± 66 cells) compared to all other masks (0-3 cells and 0-15 cells, respectively; p ≤ 0.016). Surgical and three-layer masks had significantly worse filtration of 1.1 μm aerosols than for other aerosol sizes. Wearing a mask reduced the transmission of bacteria during speech, but the inner surface of these masks became contaminated with 528-3060 bacterial cells.

Face masks effectively reduce the transmission of microbes in laboratory tests. Face masks made with seven layers were very effective at stopping transmission of S. aureus in all aerosol particle sizes. However, face masks become rapidly contaminated during wear. If masks are to be re-used, they should be regularly replaced or appropriately washed.